The production of dry solvents from raw aqueous mixtures is often costly and complicated. The preparation of dry ethanol is a good example. In the conventional process, the raw fermentation broth is stripped under moderate vacuum in a beer still. Overhead vapor from the beer still is sent to a rectification column that produces an overhead product close to the azeotrope (about 93 wt % ethanol) and a bottoms product, which is essentially water. The condensed product from the top of the column is evaporated under pressure and fed to a molecular sieve dryer, which produces ethanol of 99 wt %+purity. Such a process consumes almost 100 million Btu/h to produce 50 million gallons per year of purified ethanol from a feed containing about 11 wt % ethanol.
It is known to use two distillation columns in series to separate mixtures such as organic/water mixtures. Such processes are taught in U.S. Pat. Nos. 4,539,076; 5,035,776; and 7,297,236, for example.
It is also known to use membrane separation to treat the overhead stream from a column. Such processes are taught in U.S. Pat. No. 4,978,430; in U.S. Published Application number 2006/0070867; in Japanese Published Application number JP7227517; and in U.S. patent application Ser. No. 11/896,201.
In addition, co-owned U.S. Pat. No. 7,732,173, issued Jun. 8, 2010, to Mairal et al., teaches a process for recovering ethanol involving membrane separation, followed by dephlegmation, followed by a second membrane separation step to dehydrate the overhead stream from the dephlegmator.
U.S. Pat. No. 5,105,029 teaches the use of two columns followed by membrane separation; and U.S. Pat. No. 4,405,409 discloses the use of two membrane separation steps in series to treat a column overhead.
Specific membranes for use in dehydration of organic compounds are taught in co-owned U.S. Pat. No. 8,002,874, issued Aug. 23, 2011, to Huang et al., and co-owned and copending U.S. application Ser. No. 11/897,675.
Despite the extensive efforts represented by the prior literature, there remains a need for a process that is both energy efficient and cost effective for producing high purity dehydrated solvents, especially ethanol.